United States Homeland security requirements set forth a need for devices capable of sensitive detection of gamma rays originating from hidden radioactive material (e. g. ANSI N42.32). Commercially available high sensitivity portable or mobile gamma radiation meters can quickly detect small increases in the strength of a gamma radiation field. Some of these conventional radiation detection instruments simply display the number of detected gamma rays sensed (i.e., counted) by the device, while other conventional radiation detection devices are capable of measuring and displaying the dose rate of the gamma radiation field detected by the device. Operators of such device can set alarm thresholds on absolute numbers of the detected particles per time unit or on the measured dose rate, depending upon the device used. Alternatively, some conventional radiation detection systems are the increase of one of these quantities (gamma count or dose rate) compared to a learned background level of radiation in order to generate an alarm when the count or dose rate exceeds a predetermined threshold.